Automatically add friends or my email contacts to my facebook group - repost

I have been added several times to facebook groups I do not even know. For curiosity I went on them and saw that some of these had even over a million group members. The only explanation for me is that they use some sort of robot to add members to these groups. How do they do it? Can you develop a mechanism that can add people to my facebook page without someone manually being involved? I have a list of contacts but adding/inviting them manually would be too tedious. So I would like to have a script that allows me to add all my friends on facebook to my facebook group and/or my email contacts (maybe recognizing if that email is a facebook user) to my group. Thank you